The present invention relates to a process for preparing cyclic compounds having at least eight carbon atoms and at least one keto group, to the cyclic compounds obtained by this process and to the use thereof, in particular as fragrance or for providing a fragrance.
There is a need for effective processes for preparing cyclic compounds based on medium and specifically based on large rings which have at least one keto group. Medium rings generally have 8 to 11 carbon atoms, above 12 carbon atoms one talks of large rings, and compounds based on large rings are also referred to as macrocyclic compounds. Macrocyclic ketones, lactones and epoxides as well as further functionalized macrocycles are aroma chemicals valued in the fragrance industry. Important representatives are e.g. muscone (1), (E/Z)-8-cyclohexadecenone (2), cyclohexadecanone (3) or 9-hexadecen-16-olide (4).

The known processes for preparing cyclic ketones are generally associated with considerable synthetic expenditure. Thus, A. S. Williams gives, in Synthesis 1999, 10, 1707-1723, an overview of the synthesis of macrocyclic musk compounds. For example, the preparation of muscone (1) takes place according to this in a multistage synthesis starting from cyclododecanone by ring expansion.
DE 101 42 032 A1 describes the preparation of cycloalkadienes from cycloalkamonoenes, cyclopolyenes, acyclic polyenes or mixtures thereof by metathesis reaction in the presence of an Re2O7/γ-Al2O3 catalyst. Specifically, the preparation of 1,9-cyclohexadecadiene from cyclooctene is described.
B. D. Mookherjee et al. described in J. Org. Chem. 36, 22 (1971), 3266-3270 the synthesis of racemic muscone and cyclopentadecanone (exalton) starting from 1,9-cyclohexadecadiene. The synthesis comprises a complex reaction sequence which comprises inter alia an epoxidation step and a rearrangement step.
EP 0322537 A2 describes a process for preparing cyclic ketones by isomerization of epoxides in a polar solvent in the presence of alkali metal or alkaline earth metal halides.
U.S. Pat. No. 5,936,100 describes the synthesis of functionalized macrocycles by ring-closure metathesis. Thus, for example, the synthesis of 9-hexadecen-16-olide (D) takes place by ring-closure metathesis of 10-undecenylic acid 5-hexenyl ester. However, the starting material diene is only poorly available on an industrial scale.
Paul R. Story and P. Busch describe in “Modern methods for the synthesis of macrocyclic compounds”, Adv. Org. Chem. 1972, 8, 67-95 diverse synthesis routes, inter alia by means of ring expansion, ring reduction, 1,2-cycloaddition, etc. The so-called “Story synthesis” comprises here the formation and destruction of corresponding peroxides, which is associated with correspondingly high synthetic and safety expenditure.
The known processes for preparing cyclic compounds having at least eight, specifically at least 12, ring carbon atoms are either multistage syntheses or start from starting materials that are only poorly available on an industrial scale.
Cyclododecanone is an important intermediate for preparing lauryl lactam, dodecanedicarboxylic acid and polyamides derived therefrom. To prepare cyclododecanone, 1,5,9-cyclododecatriene can be subjected to an oxidation, specifically an oxidation with N2O, the main product obtained being cyclododeca-4,8-dienone, which is then subjected to a selective hydrogenation to give cyclododecanone. Processes of this type are described for example in WO 2005/030690, WO 2008/000754 and WO 2010/086314.
Surprisingly, it has now been found that, starting from cyclododeca-4,8-dienone, it is possible to prepare cyclic compounds having at least eight carbon atoms and at least one keto group simply and effectively by means of metathesis reaction. Here, specifically the preparation of a metathesis product which has a high fraction of macrocyclic ketones and diketones of ring sizes C16, C20, C24 and higher homologues having in each case four additional carbon atoms is possible. This metathesis product is suitable, optionally after separation into fractions or essentially pure compounds, as fragrance or for providing a fragrance. Moreover, this process is particularly advantageous since cyclododeca-4,8-dienone is industrially available in large amounts as intermediate of the preparation of cyclododecanone. Furthermore, only one synthesis step is required in order to obtain a product which can have a large number of interesting compounds. Thus, the metathesis products obtainable by the process according to the invention or fractions or essentially pure compounds therefrom can also serve as valuable intermediates. They are suitable for further processing e.g. by hydrogenation, Baeyer-Villiger oxidation to give macrocyclic lactones, synthesis building blocks of diverse specialty chemicals and fragrances, etc.